NAME

SYNOPSIS

DESCRIPTION

mbind() sets the NUMA memory policy, which consists of a policy mode
and zero or more nodes, for the memory range starting with addr and
continuing for len bytes. The memory policy defines from which node
memory is allocated.
If the memory range specified by the addr and len arguments includes an
"anonymous" region of memory—that is a region of memory created using
the mmap(2) system call with the MAP_ANONYMOUS—or a memory mapped file,
mapped using the mmap(2) system call with the MAP_PRIVATE flag, pages
will only be allocated according to the specified policy when the
application writes [stores] to the page. For anonymous regions, an
initial read access will use a shared page in the kernel containing all
zeros. For a file mapped with MAP_PRIVATE, an initial read access will
allocate pages according to the process policy of the process that
causes the page to be allocated. This may not be the process that
called mbind().
The specified policy will be ignored for any MAP_SHARED mappings in the
specified memory range. Rather the pages will be allocated according
to the process policy of the process that caused the page to be
allocated. Again, this may not be the process that called mbind().
If the specified memory range includes a shared memory region created
using the shmget(2) system call and attached using the shmat(2) system
call, pages allocated for the anonymous or shared memory region will be
allocated according to the policy specified, regardless which process
attached to the shared memory segment causes the allocation. If,
however, the shared memory region was created with the SHM_HUGETLB
flag, the huge pages will be allocated according to the policy
specified only if the page allocation is caused by the process that
calls mbind() for that region.
By default, mbind() only has an effect for new allocations; if the
pages inside the range have been already touched before setting the
policy, then the policy has no effect. This default behavior may be
overridden by the MPOL_MF_MOVE and MPOL_MF_MOVE_ALL flags described
below.
The mode argument must specify one of MPOL_DEFAULT, MPOL_BIND,
MPOL_INTERLEAVE or MPOL_PREFERRED. All policy modes except
MPOL_DEFAULT require the caller to specify via the nodemask argument,
the node or nodes to which the mode applies.
The mode argument may also include an optional modeflag . The
supported modeflags are:
MPOL_F_STATIC_NODES (since Linux-2.6.26)
A non-empty nodemask specifies physical node ids. Linux does
not remap the nodemask when the process moves to a different
cpuset context, nor when the set of nodes allowed by the
process’s current cpuset context changes.
MPOL_F_RELATIVE_NODES (since Linux-2.6.26)
A non-empty nodemask specifies node ids that are relative to the
set of node ids allowed by the process’s current cpuset.
nodemask points to a bitmask of nodes containing up to maxnode bits.
The bit mask size is rounded to the next multiple of sizeof(unsignedlong), but the kernel will only use bits up to maxnode. A NULL value
of nodemask or a maxnode value of zero specifies the empty set of
nodes. If the value of maxnode is zero, the nodemask argument is
ignored. Where a nodemask is required, it must contain at least one
node that is on-line, allowed by the process’s current cpuset context
[unless the MPOL_F_STATIC_NODES mode flag is specified], and contains
memory.
The MPOL_DEFAULT mode requests that any non-default policy be removed,
restoring default behavior. When applied to a range of memory via
mbind(), this means to use the process policy, which may have been set
with set_mempolicy(2). If the mode of the process policy is also
MPOL_DEFAULT, the system-wide default policy will be used. The system-
wide default policy allocates pages on the node of the CPU that
triggers the allocation. For MPOL_DEFAULT, the nodemask and maxnode
arguments must be specify the empty set of nodes.
The MPOL_BIND mode specifies a strict policy that restricts memory
allocation to the nodes specified in nodemask. If nodemask specifies
more than one node, page allocations will come from the node with the
lowest numeric node ID first, until that node contains no free memory.
Allocations will then come from the node with the next highest node ID
specified in nodemask and so forth, until none of the specified nodes
contain free memory. Pages will not be allocated from any node not
specified in the nodemask.
The MPOL_INTERLEAVE mode specifies that page allocations be interleaved
across the set of nodes specified in nodemask. This optimizes for
bandwidth instead of latency by spreading out pages and memory accesses
to those pages across multiple nodes. To be effective the memory area
should be fairly large, at least 1MB or bigger with a fairly uniform
access pattern. Accesses to a single page of the area will still be
limited to the memory bandwidth of a single node.
MPOL_PREFERRED sets the preferred node for allocation. The kernel will
try to allocate pages from this node first and fall back to other nodes
if the preferred nodes is low on free memory. If nodemask specifies
more than one node ID, the first node in the mask will be selected as
the preferred node. If the nodemask and maxnode arguments specify the
empty set, then the memory is allocated on the node of the CPU that
triggered the allocation. This is the only way to specify "local
allocation" for a range of memory via mbind().
If MPOL_MF_STRICT is passed in flags and policy is not MPOL_DEFAULT,
then the call will fail with the error EIO if the existing pages in the
memory range don’t follow the policy.
If MPOL_MF_MOVE is specified in flags, then the kernel will attempt to
move all the existing pages in the memory range so that they follow the
policy. Pages that are shared with other processes will not be moved.
If MPOL_MF_STRICT is also specified, then the call will fail with the
error EIO if some pages could not be moved.
If MPOL_MF_MOVE_ALL is passed in flags, then the kernel will attempt to
move all existing pages in the memory range regardless of whether other
processes use the pages. The calling process must be privileged
(CAP_SYS_NICE) to use this flag. If MPOL_MF_STRICT is also specified,
then the call will fail with the error EIO if some pages could not be
moved.

RETURNVALUE

On success, mbind() returns 0; on error, -1 is returned and errno is
set to indicate the error.

ERRORS

EFAULT Part or all of the memory range specified by nodemask and
maxnode points outside your accessible address space. Or, there
was an unmapped hole in the specified memory range.
EINVAL An invalid value was specified for flags or mode; or addr+len
was less than addr; or addr is not a multiple of the system page
size. Or, mode is MPOL_DEFAULT and nodemask specified a non-
empty set; or mode is MPOL_BIND or MPOL_INTERLEAVE and nodemask
is empty. Or, maxnode exceeds a kernel-imposed limit. Or,
nodemask specifies one or more node IDs that are greater than
the maximum supported node ID. Or, none of the node IDs
specified by nodemask are on-line and allowed by the process’s
current cpuset context, or none of the specified nodes contain
memory. Or, the mode argument specified both
MPOL_F_STATIC_NODES and MPOL_F_RELATIVE_NODES.
EIOMPOL_MF_STRICT was specified and an existing page was already on
a node that does not follow the policy; or MPOL_MF_MOVE or
MPOL_MF_MOVE_ALL was specified and the kernel was unable to move
all existing pages in the range.
ENOMEM Insufficient kernel memory was available.
EPERM The flags argument included the MPOL_MF_MOVE_ALL flag and the
caller does not have the CAP_SYS_NICE privilege.

VERSIONS

The mbind(), system call was added to the Linux kernel in version
2.6.7.

CONFORMINGTO

This system call is Linux-specific.

NOTES

For information on library support, see numa(7).
NUMA policy is not supported on a memory mapped file range that was
mapped with the MAP_SHARED flag.
The MPOL_DEFAULT mode can have different effects for mbind() and
set_mempolicy(2). When MPOL_DEFAULT is specified for set_mempolicy(2),
the process’s policy reverts to system default policy or local
allocation. When MPOL_DEFAULT is specified for a range of memory using
mbind(), any pages subsequently allocated for that range will use the
process’s policy, as set by set_mempolicy(2). This effectively removes
the explicit policy from the specified range, "falling back" to a
possibly non-default policy. To select explicit "local allocation" for
a memory range, specify a mode of MPOL_PREFERRED with an empty set of
nodes. This method will work for set_mempolicy(2), as well.
Support for huge page policy was added with 2.6.16. For interleave
policy to be effective on huge page mappings the policied memory needs
to be tens of megabytes or larger.
MPOL_MF_STRICT is ignored on huge page mappings.
MPOL_MF_MOVE and MPOL_MF_MOVE_ALL are only available on Linux 2.6.16
and later.